This disclosure describes a system and method directed toward directional drilling of a subterranean well and, in particular, controlling the angle and direction of drilling through selectable bending of a shaft within a steering sub connected to the drill bit.
In some conventional drilling operations, a mud motor is used to rotate the drill bit with respect to the drill string. A typical mud motor is a positive displacement motor that is driven by the flow of drilling fluid, commonly known as “mud,” that is pumped down from the surface through the mud motor and then to the drill bit, where the drilling fluid flows into the borehole through jets in the drill bit. The drilling fluid flushes rock cuttings and debris from the cutting face of the drill bit and carries them to the surface.
It is sometimes desirable to directionally drill at an angle or even horizontally away from a vertical line that is directly underneath a drilling rig. One conventional method of directional drilling is to provide a small bend angle above the mud motor and the bearing assembly that supports the drill bit. If the drill string is rotated from the surface while drilling, the drill bit creates a straight, slightly oversized borehole. In the absence of surface drill string rotation and only rotation from the mud motor, however, the drill bit will advance in the direction of the bend and create a borehole that curves away from the vertical axis in the direction of the bend.
One drawback of the conventional method of directional drilling is that the rotational position of the lower end of a long drill string may not be precisely known due to elastic rotational deformation of the drill string between the surface and the mud motor. This uncertainty may result in the drill bit progressing in a lateral direction other than the intended direction, requiring an adjustment in the rotational position of the drill string to attempt to steer the drill bit back toward the intended direction.
An additional drawback of the conventional method of directional drilling is that the speed of drilling the straight portions of the borehole, which may form the majority of the length of a typical borehole, with a drill string having a bent sub is reduced compared to drilling with a drill string not having a bent sub because the borehole must be larger in diameter due to the necessary rotation of the drill string to maintain a straight drill path with the angled drill bit.